Most people would describe the Immersive Armors as lore-friendly, mostly keeping to the design of the original game; but the mod also includes Viking and Samurai armor, neither of which exist in TES lore. So where do we draw the line of being lore-friendly?

Viking and nords have similar outfits I would say. And samurai armor could just be old blade armor, or Akaviri armor.
Anyway, I think lore friendly would mean that you could explain it using the lore, or you could explain it with a different story that won't conflict with the lore in a very obvious way

I suppose. I think the main thing I disagree with is the naming structure; calling it Samurai or Viking armor is projecting our own world into theirs, creating factions that shouldn't exist there. I think it would make more sense if it were called something like "Akaviri warlord armor", or "Nord raider armor".

I'd say something that looks like it belongs in Skyrim, not lifted directly from another source or obviously inconsistent with the gritty-ish, visually realistic fantasy setting. The Viking and Samurai armor names are kind of iffy to me too, but at least they look like they belong.

ES Lore isn't some iron fist to beat everybody else over the head with. It's just a description of a worldspace to ensure some consistency to allow people to imagine a functioning world. It's a lot like adapting a book. If you copy it out word-for-word, the result may very well be rather boring, whereas if you change a few things that you felt were less than perfect in the source text, your adaptation might be all the better for it. Change too much, and you might as well not bother to use it in the first place. It's a very good idea to familiarize yourself as far as possible with the information available e.g. at The Imperial Library. Only then can you make the decision as to how far you agree with the decisions Skyrim's developers made.

TES Lore offers much room for expansion and interpretation without breaking it and just about anything can be done tactfully if you research enough.

Lore-friendly means that within the context of believable extrapolation, an addition to the game in the means of resources or character does not break the rules and boundaries of the logical in consideration to the lore in canon.

For example, the extrapolation of Akaviri meaning "Samurai" or Eastern-inspired themes is only loosely explored, but not specifically defined despite using Eastern-inspired weapons and some armor bits. The easiest IRL comparison is of course Samurai so lots of modders try to fit the art and wear style of TES into the idea behind the old Samurai warriors; this is how we end up with "Akaviri" suggested as mods revolving around Samurai.

How lore-friendly this ends up is entirely up to debate, but the inclusion of actual or borrowed ideas of established outside canon (say a Naruto reference or something) would no longer be attempting to limit a mod to the established boundaries of the canon. With a distinct art style, locational information, basic background, and available comparative resources it's safe to say that there are some pretty "safe" bets that lots of well-thought "Akaviri" additions could conceivably be lore-friendly.

Things and ideas that come from lore but do not follow it, such as the idea behind the Dwemer Exoskeleton, are not lore-friendly. While made of entirely in-game resources and using attacks and patterns of the centurions for the PC, the Dwemer would never have built such a device to house man or mer as they isolated themselves and scorned them openly. Lore-wise, the reason we "know" (in the in-game context) so little about the dwarves is due to this pattern. Some convincing stories could be made up, but the established doctrine is one of true isolation.

The moral ends up being the strength and familiarity with established lore guiding the development of the mod. When outside or unlikely/impossible ideas start to become introduced, the mod is no longer Lore Friendly.

I'm with you up until the exoskeleton. While I agree that it is not lore friendly (and wouldnt add that mod myself) I just need to point out that the dwemer were comperable in size to men and elves, so there's no reason a suit build for them wouldn't fit men and elves.